The present invention relates to microlithography as used to transfer a pattern, defined by a reticle or mask, onto a substrate such as a semiconductor wafer. Microlithography methods and apparatus are used extensively in the manufacture of semiconductor integrated circuits, memories, high-resolution displays, and the like. More specifically, the invention pertains to methods and apparatus for cleaning a reticle or mask, or a substrate such as a semiconductor wafer, to remove foreign particulate matter adhering thereto.
In microlithography of a fine circuit pattern, as defined by a reticle, onto a wafer or other substrate, the high pattern resolution now required makes any particulate contamination on the reticle or wafer intolerable. I.e., particulate contamination typically causes defects in the fine circuit pattern as projected onto the wafer. Such defects can impair the function of the semiconductor device or display, resulting in product rejects.
Currently, foreign matter adhering to a substrate or reticle is usually removed by wet cleaning using a liquid exhibiting a chemical effect on the foreign matter. However, such wet cleaning typically requires multiple off-line steps to execute. The resulting increased handling increases the probability of re-contamination or damage and can substantially reduce throughput. Hence, considerable effort is being expended to develop practical xe2x80x9cdryxe2x80x9d cleaning methods for reticles and wafers.
One example of a conventional dry cleaning method is disclosed in Japanese laid-open patent publication no. Hei 6-95510, in which small particles are removed from a reticle by irradiating the reticle with pulsed laser light. The pulsed laser light serves to either vaporize the particles adhering to the reticle surface or remove the particles from the reticle surface. The latter is achieved by accelerating the particle, relative to the reticle, due to a rapid laser-induced thermal expansion of the particle relative to the reticle surface.
As disclosed in Tam et al., Journal of Applied Physics 71:3515-3523 (1992), a film of water is applied to a surface of a sample (e.g., wafer or reticle) requiring cleaning. By locally irradiating the water film with a pulse of laser light, the water film in the vicinity of the incident laser light is evaporated from the sample surface. The resulting sudden and explosive localized production of water vapor helps dislodge contaminant particles from the sample surface in the vicinity of the incident laser light, which tends to enhance the cleaning effect.
Unfortunately, conventional laser-cleaning systems utilizing a surficial water film exhibit certain problems. One problem concerns the atmosphere of water vapor through which the laser beam must pass as the beam propagates from the laser source to the sample. Since the laser produces a wavelength that is absorbed by the water film, passage of the laser beam through the atmosphere including water vapor substantially reduces the intensity of the laser beam and degrades cleaning efficacy. If the amount of water in the atmosphere traversed by the laser beam varies, then the intensity of the laser light with which the sample is irradiated correspondingly varies. Consequently, cleaning is inconsistent.
Another problem with conventional laser-cleaning systems is a difficulty in ascertaining whether the target particulate contaminant has actually been removed during cleaning. In conventional practice, inspecting whether a cleaning step has been effective is determined in an off-line inspection step after performing the cleaning step. Unfortunately, if the cleaning step was not effective, the sample must be returned to have the cleaning step repeated, after which the sample must be inspected again. Such a protocol substantially reduces throughput and increases production costs.
Conventional laser-cleaning systems also achieve inconsistent cleaning because, in general, the smaller the particle of contaminant matter adhering to the sample, the greater the tenacity with which the particle adheres to the sample. Cleaning efficacy can be improved by increasing the intensity of the laser light; however, increased intensity increases the risk of damaging the sample itself.
In view of the shortcomings of the conventional art as summarized above, an object of the present invention is to provide improved methods and apparatus for removing particles of foreign matter from the surface of a sample (such as a semiconductor wafer) using pulses of laser light, wherein propagation of the laser light from a laser source to the sample is not accompanied by any significant decrease in the intensity of the light.
According to a first aspect of the invention, methods are provided for removing particles of foreign matter from the surface of a sample. According to a first representative embodiment, a film of water is formed on the surface of the sample. A propagation path is provided for laser light propagating from a laser source to the sample surface. Provided in the propagation path is an environment exhibiting essentially no absorption of the laser light at a wavelength at which water would absorb the laser light. Such an environment is termed herein a xe2x80x9cdryxe2x80x9d environment. A pulse of the laser light, having a wavelength that is absorbed by water, is directed from the laser source through the propagation path to a locus on the surface of the sample, wherein the locus is a location on the surface of the sample at which the particle of foreign matter is attached to the sample surface.
The environment in the propagation path can include a gas. According to one example, the gas is a single gas or gas mixture that lacks water vapor. According to another example, the gas is an inert gas or mixture of such gases. Alternatively, the environment in the propagation path can be a vacuum.
According to another aspect of the invention, apparatus are provided for removing particles of foreign matter from a surface of a sample. A first representative embodiment of such an apparatus comprises a pulsed laser light source that produces pulsed laser light having a wavelength absorbed by water. An applicator is situated and configured to form a film of water on the surface of the sample. In a propagation path extending from the laser light source to the sample is provided a propagation environment for the laser light that exhibits essentially no absorption of the laser light at a wavelength at which water would absorb the laser light. Such an apparatus can include an optical system situated relative to the laser light source and configured to irradiate a locus on a sample with the pulsed laser light, wherein the locus includes a particle of foreign matter adhering to the sample. The propagation environment can be a vacuum. Alternatively, the propagation environment can include a gas or gas mixture that lacks water vapor. For example, the gas can be an inert gas or mixture of such gases.
The propagation path desirably comprises a beam conduit. If the propagation environment is a vacuum (e.g., about 10 Pa pressure), then the beam conduit can be connected to a vacuum pump to create the vacuum in the beam conduit.
If the propagation environment includes a gas, then the beam conduit can be connected via a gas-supply tube or analogous structure to a gas supply.
By way of example, the beam conduit can be filled with nitrogen gas which is effective in displacing and excluding water molecules from the propagation environment.
Another object of the invention is to provide methods and apparatus for removing foreign matter from the surface of a sample, wherein the cleaning and inspection steps are conducted simultaneously, desirably in a xe2x80x9cdryxe2x80x9d environment. According to one aspect of the invention directed to such an object, methods are provided for removing a particle of foreign matter from a surface of a sample. According to a representative embodiment of such a method, a pulse of laser light is directed from a laser light source to a locus on the surface of the sample. While directing the pulse of laser light in such a manner, the locus is observed. Thus, there is no need for the cleaning step and inspection step to be performed separately. Also, there is no need for the cleaning and inspection steps to be repeated because the state actual cleaning is observable in real time, thereby increasing productivity and lowering manufacturing costs associated with devices made from the sample.
This method can include the step of forming a film of water on the surface of the sample. A propagation path can be provided for laser light propagating from a laser source to the sample surface. In the propagation path can be provided an environment exhibiting essentially no absorption of the laser light at a wavelength at which water would absorb the laser light.
According to another aspect of the invention, apparatus are provided for removing a particle of foreign matter from a surface of a sample. A representative embodiment of such an apparatus comprises a pulsed laser light source that produces pulsed laser light. An optical system is situated relative to the laser light source and configured to irradiate a locus on a sample with the pulsed laser light. The apparatus also includes an observation system situated relative to the sample and configured for observing the locus of the workpiece as the locus is being irradiated with the pulsed laser light. The observation system can comprise an environmental scanning electron microscope (ESEM). An ESEM is especially advantageous because observations can be performed therewith at high accuracy and precision due to the high resolution obtainable using an ESEM.
Such an apparatus can also include an applicator situated and configured to form a film of water on the surface of the sample. Such an apparatus can also include a propagation path extending from the laser light source to the sample. As summarized above, the propagation path provides a propagation environment for the laser light that exhibits essentially no absorption of the laser light at a wavelength at which water would absorb the laser light.
Another object of the invention is to provide methods and apparatus for removing particles of foreign matter from the surface of a sample, wherein the intensity of the laser light can be maintained sufficiently low to avoid damaging the surface of the sample. To such end and according to another aspect of the invention, methods are provided that include the step of directing a pulse of laser light from a laser light source to a locus on the surface of the sample. While performing such irradiation of the locus, the sample is vibrated. Such vibration facilitates dislodgment of the particles from the sample surface and thereby allows a lesser intensity of laser light to be used than would otherwise be necessary if the sample were not vibrated. Such methods can also include the step of observing the locus while irradiating the locus. Also, such methods can include the steps of forming a film of water on the surface of the sample, and passing the laser light (as it propagates from a source to the sample) through a propagation path containing an environment exhibiting essentially no absorption of the laser light at a wavelength at which water would absorb the laser light.
According to another aspect of the invention, apparatus are provided that achieve vibration of the sample as the sample is being irradiated with pulses of laser light. A representative embodiment of such an apparatus comprises a pulsed laser light source that produces pulsed laser light. An optical system is situated relative to the laser light source and configured to irradiate a locus on a sample with the pulsed laser light. The apparatus also includes a sample vibrator situated and configured to vibrate the sample as the locus on the surface of the sample is being irradiated with the pulsed laser light. Such an apparatus can also include an applicator situated and configured to form a film of water on the surface of the sample, and can include a propagation path that provides a propagation environment for the laser light that exhibits essentially no absorption of the laser light at a wavelength at which water would absorb the laser light.